# Misunderstanding of kinetic energy turning into heat

Imagine a block sliding on a surface including friction. Gradually, it'll stop. If we touch both the surface and the block, they are hotter, their temperatures have increased. I've read that kinetic energy has turned into heat, which doesn't sound accurate to me. I handle the next heat definition that is very common: Heat is the transfer of energy due to a diference of temperatures between to two systems. Regarding only about the fact that heat is a flow of energy, so it doesn't appear correct to argue kinetic energy is now heat, since kinetic energy is something you have, but heat isn't. I read a very nice answer that holds this: kinetic energy is transfered as heat, and now is thermal energy. Thermal energy has to do with internal energy. But internal energy may depend on very variables, but thermal energy is only related with temperature, right? Yet I find it confusing. Anyway, so, the total energy can be distributed to increase the temperature of both objtects in many ways, right? Does the same thing happen with collitions?

On the other hand, would it be equivalent saying the work that friction performs is now thermal energy? Finally, since heat is transfer of energy, (any kind of energy), if two systems are in contact, one hotter than the other, transfer of energy takes place, what kind of energy is transfered?. That's it.

The confusion comes between concepts of thermodynamics, and concepts of mechanics. Temperature was originally defined as a thermodynamic variable, but it can be shown that it emerges from the underlying molecular structure of matter. Studying the kinetic theory of gases one can get an intuition on how thermodynamic variables emerge from the microscopic world of atoms and molecules.

It can be shown that temperature, emerges from an average over the kinetic energy of the individual molecules bouncing against each other in the volume of the gas.

$$\large{T= \frac{m\bar{v^2}}{3k_B}}$$

When the gas is heated, the average kinetic energy of the molecules in the gas goes up.

Solids are composed of atoms and molecules, they are not as free as gas molecules to absorb kinetic energy, but the molecules do have vibrational and rotational degrees of freedom and the kinetic energy of friction can be transferred there and macroscopically appear as higher temperature due to the higher "kinetic" energy of individual molecules.

On the other hand, would it be equivalent saying the work that friction performs is now thermal energy?

Microscopically it is higher vibration and rotation rates of the molecules , macroscopically it is heat energy transferred to raise the kinetic energy individual molecules .

Finally, since heat is transfer of energy, (any kind of energy), if two systems are in contact, one hotter than the other, transfer of energy takes place, what kind of energy is transfered?

Heat is also transferred by the radiation of the bodies, electromagnetic radiation, called black body radiation. This is absorbed from the higher temperature body to the lower, raising the vibrational and rotational levels in solids (and the kinetic energy of the molecules in gases and liquids) and thus the temperature.

• Thank you for your time. Just some questions. Regarding the last question, as well as the kinetic energy turn into energy for molecules tu vibrate (therefore increasing the temperature), if we mix ice with liquid water, of course the ice will get hotter, but what kind of energy has been transfered?
– Omar
Oct 30, 2015 at 1:48
• The liquid water will transfer radiative(black body) energy to the bulk ice initially, and at its surfaces kinetic energy of the liquid molecules will scatter and increase the energy to the frozen ones. That is why the surfaces melt faster, macroscopically it is called convection which is generated and increased because of the temperature differences between ice and water. Oct 30, 2015 at 4:49
• Thank you for your patience again. I've learnt something new.
– Omar
Oct 30, 2015 at 14:31